The present invention relates to a terrestrial/cable broadcasting receiver for receiving a signal transmitted via a terrestrial or cable broadcasting channel through one receiver in a high definition television (HDTV).
The U.S. HDTV transmission method, which is divided into terrestrial broadcasting and cable broadcasting, was determined by Grand Alliance. For terrestrial broadcasting, the 8-vestigial side band (VSB) the number of whose symbol level is eight, was selected. For cable broadcasting, 16-VSB the number of whose symbol level is sixteen was chosen.
However, the terrestrial broadcasting and cable broadcasting are different in transmission method. 8-VSB, which is for terrestrial broadcasting, transmits one program within 6 MHz, 16-VSB sending two programs within 6 MHz.
In terrestrial broadcasting, a broadcasting signal is transmitted via free spaces as a medium so that it is exposed to various noises present in the spaces and its attenuation becomes severe. Especially, there is created common-channel interference or nearby-channel interference by broadcasting signals emitted from nearby broadcasting stations.
The common-channel interference by an NTSC broadcasting station is hard to control with a general band-limiting filter. For this reason, only an NTSC carrier is removed by using an NTSC rejection filter, which, however, causes a slight damage in an intended signal.
A multipath interference, which is caused when a broadcasting signal is mixed with the same signal reflected by high objects such as mountains and buildings, is removed by a highly-integrated channel equalizing filter. Accordingly, the terrestrial broadcasting uses a very complicated receiver, and is difficult to transmit at a high data rate.
Meanwhile, in the cable broadcasting, a signal is transmitted through a coaxial cable so that there is present the nearby-channel interference due to nearby-frequency-band transmission data. However, there is no common-channel interference. Therefore, the NTSC rejection filter which damages an intended signal is unnecessary for the cable broadcasting.
In addition, since the cable broadcasting has no multipath interference, a channel equalizing filter can be employed, which compensates only for linear distortion produced in a transmission/reception device and is simple in construction, and has a fast convergence velocity.
Furthermore, since random noise is seldom mixed in the cable channel, unlike the terrestrial channel, the cable broadcasting makes high-level symbol transmission possible.
Although the terrestrial channel transmits only two-bit data for every symbol, the cable channel can transmit four-bit data for every symbol so that two programs are transmitted within 6 MHz.
In order to increase immunity from random noise, the terrestrial channel uses the Trellis coded modulation (TCM) with complicated hardware in detecting a signal. However, even without the TCM the cable channel has a high immunity from random noise so that signal detection can be performed with a discriminator which is simple in hardware.
As discussed above, the terrestrial broadcasting receiver and cable broadcasting receiver are different in configuration, respectively shown in FIGS. 1 and 2.
FIG. 1 is a block diagram of a conventional terrestrial broadcasting receiver for HDTV, with FIG. 2 showing a conventional cable broadcasting receiver for HDTV.
The conventional terrestrial broadcasting receiver for HDTV comprises an antenna 1, tuner 2, channel selector 3, IF filter/sync detector 4, sync extractor 5, NTSC rejection filter 6, NTSC interference discriminator 8, multiplexer 9, channel equalizer 10, phase tracker 11, TCM decoder 12, data deinterleaver 13, R-S decoder 14, and data derandomer 15.
The operation of the terrestrial broadcasting receiver will be explained below.
A signal input via antenna 1 is converted into an IF signal in tuner 2 according to a channel selected by channel selector 3.
The thus converted IF signal is changed into a form suitable for detection in IF filter/sync detector 4, and becomes a baseband signal. In sync detection, a carrier extracted in sync extractor 5 is used.
The baseband signal output from IF filter/sync detector 4 passes through NTSC rejection filter 6 when NTSC interference discriminator 8 detects that there is NTSC interference. When there is no NTSC interference, the baseband signal is directly output not via NTSC rejection filter 6. For this selection, multiplexer 9 is used which is switched under the control of NTSC interference discriminator 8.
For the terrestrial broadcasting output from multiplexer 9, multipath noise is fatal to signal detection. Therefore, in order to cancel ghost error due to multipath, the terrestrial broadcasting is channel-equalized in channel equalizer 10 using a highly integrated filter of large number of filter taps. However, the large number of filter taps increases the convergence time of channel equalizer 10, lengthening the stabilization time of the whole system accordingly.
In restoring the carrier for sync detection of sync detector 5, the phase may deviate slightly. Phase tracker 11 finely adjusts the deviating phase of the signal formed after the channel equalization in channel equalizer 10.
After the phase tracking, the baseband signal output from phase tracker 11 is symbol-detected through TCM decoder 12. TCM decoder 12 simultaneously performs detection and error-correction in order to increase immunity from random noise which may be produced on the channel.
The symbol data detected in TCM decoder 12 passes through data deinterleaver 13 and R-S decoder 14. In these circuits, burst errors which are not corrected in TCM decoder 12 and appear in groups due to the large volume of random noise are corrected.
The data error-canceled through data deinterleaver 13 and R-S decoder 14 is restored to the original data in data derandomer 15, and input to the video and audio decoder terminals.
In other words, since data in a predetermined pattern affects the performance of sync extractor 5 and channel equalizer 9, the data is randomized on the transmission side and restored to the original form in data derandomer 15.
Referring to FIG. 2, the conventional cable broadcasting receiver for HDTV comprises an antenna 20, tuner 21, channel selector 22, IF filter/sync detector 23, sync extractor 24, channel equalizer 25, phase tracker 26, discriminator 27, data deinterleaver 28, R-S decoder 30, and data derandomer 31.
Since HDTV cable broadcasting has no NTSC interference and very little random noise, the HDTV cable broadcasting receiver does not require NTSC rejection filter 6 used in the HDTV terrestrial broadcasting receiver. In addition, TCM decoder 12 used in the HDTV terrestrial broadcasting receiver may be replaced with simple discriminator 27. Since NTSC rejection filter 6 is not used, NTSC interference discriminator 8 and multiplexer 9 are used, neither.
In the cable broadcasting, two programs are transmitted in one band so that selection of one between two programs is necessary along with tuning. This function is performed in data deinterleaver 29, requiring no additional component in the HDTV cable broadcasting receiver.
The operation of the HDTV cable broadcasting receiver constructed as above will be described below.
A signal input via antenna 20 is converted into an IF signal in tuner 21 according to a channel selected by channel selector 22.
The thus converted IF signal is changed into a form suitable for detection in IF filter/sync detector 23, and becomes a baseband signal. In sync detection, a carrier extracted in sync extractor 24 is used.
The baseband signal output from IF filter/sync detector 23 is channel-equalized in channel equalizer 25 in order to cancel ghost error. A phase which may deviate slightly in restoring the carrier for sync detection of sync detector 24 is finely adjusted in phase tracker 26. Since cable broadcasting has less ghost error caused in multipath, unlike terrestrial broadcasting, channel equalizer 25 which contains a small number of filter taps is used.
After the phase tracking, the baseband signal output from phase tracker 26 is symbol-detected through discriminator 27. In cable broadcasting random noise which may be created on the channel is very little so that simple discriminator 27 can be only used instead of the TCM decoder.
The symbol data detected in discriminator 27 passes through data deinterleaver 28 and R-S decoder 30. In these circuits, burst errors which are not corrected in discriminator 27 and appear in groups due to the large volume of random noise are corrected. At the same time, out of two programs transmitted from one band by channel selector 22, one program is selected in data deinterleaver 28.
The data error-canceled through data deinterleaver 28 and R-S decoder 30 is combined to the original data in data derandomer 31, and input to the video and audio decoder terminals.
In order to receive both the HDTV terrestrial broadcasting and cable broadcasting, terrestrial broadcasting receiver and cable broadcasting receiver both are required in the conventional system.